It has previously been proposed to control current flow through a load, particularly incandescent lamps, relay coils, motors, or the like by controlling conduction of a power transistor, or another power semiconductor element. If the load is essentially a low impedance load, having one terminal connected to a supply source which is subject to voltage surges, protecting the integrated circuit by an integrated Zener diode is difficult.
Zener diodes as voltage protecting devices are well known; these diodes break down when the predetermined voltage is applied thereto, thus limiting overvoltage peaks. Such circuits cannot always be used in integrated circuit (IC) networks, or may be suitable therefore. If excess voltages occur during a period of time which is more than a sudden brief excess pulse, then integrated Zener diodes cannot be used since the thermal capacity of the housing for the IC chip must then be dimensioned to be capable of dissipating the power in the Zener diode upon breakdown. This power is substantially higher than the normal operating power, and hence the IC chip will heat excessively. Excessive heating of the chip will also damage, or destroy the power transistor, so that it will, thermally, break down.